Apparatus for sensing levels in containers holding liquids



United States Patent [72] Inventor Walter Holler Meersburg (Bodenseel.Schutzenrain. Germany [211 Appl No. 785,457

[22] Filed Dec. 18, 1968 Division 01" Ser. No. 550,355, May 16, 1966,Patent No. 3335.680.

[45] Patented Dec. 15, I970 [S4] APPARATUS FOR SENSING LEVELS INCONTAINERS HOLDING LIQUIDS 3 Claims. 7 Drawing F lgs. [52] 11.8. CI .1137/392; 73/304 [51] Int. Cl E03c 1/242 Primary Examiner-William F. ODeaAssistant Examiner David R. Matthews Attorney-Harry Ernest RubensABSTRACT: Apparatus for sensing the rate of change of temperature due tochanging liquid levels within a container. such as a washing machine,which rate of change is transformed into an electrical signal in turnapplied to a response apparatus for switching and control purposes.

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WALTER HoLzEQ gnu $111M Attorney APPARATUS FOR SENSING LEVELS INCONTAINERS HOLDING LIQUIDS The present invention relates to apparatusfor sensing levels in containers holding liquids, particularly washingmachines and the like, where the variable ofstate ofa sensing means, forexample its greater temperature in relation to the temperature of theliquid, changes irregularly as the level of liquid to be sensedapproaches and reaches the sensing means. The application is a divisionof my application Ser. No. 550,355 filed May 16, 1966 and allowed withmethod claims now US. Pat. No. 3,435,680.

It is the object of the invention to increase the accuracy of and reducethe period of time necessary for the sensing of liquid levels.

It is already known to dispose a heated, thermally variable resistancein the liquid container in such a way that when the predetermined headof liquid is attained, the aforesaid resistance is cooled by the liquidso that its resistance value drops below a threshold value, tripping thecontrol operation which terminates filling. However, the heat transferfrom the liquid to the resistance of the sensing means is not constant,due to deposits of lime and dirt, so that a variable thickness ofinsulant is present on the sensing means which can absorb considerablechanges in temperature, thus not only is the time required for theresistance to cool to the actuating temperature prolonged, but thetemperature differential between the heated resistance and the liquid isreduced in an indefinite way. Furthermore, the heating output requiredto heat the resistance is naturally limited. Any drops ofliquidremaining, for instance, from a previous filling, are also heated,therefore, which is particularly disadvantageous if such sensing meansare located close to the bottom of the vessel and are used as a boildryprotection because, by the time the resistance value has dropped to thepreset threshold value, considerable time, for example 2 minutes mayelapse, during which time the already empty container continues to beheated to an undiminished extent and may be damaged or destroyed as aresult. The solution of increasing the temperature of the uncooledresistance, thus increasing the differential in respect of the liquidtemperature sufficiently for the aforesaid influence to remain withintolerable limits, has, on structural grounds, proved to be difficult ornot feasible.

Where such an unheated resistance is used as a temperature sensingmeans, then the inevitable deposits similarly cause an indefinitethermal delay and a corresponding falsification of the liquidtemperament measurement. Such an arrangement is associated with all thedeficiencies which occur with absolute measurement. In many cases,however, these disadvantages are taken into account because, on economicgrounds, it would be unjustified to incorporate a further temperaturesensing means. Also, the known scanning of liquid levels by means of atleast one electrode inserted in insulated manner in the container wallhas the disadvantage of being intensely dependent on dirt or limedeposits. If the conductance or the capacitance is used as thecharacteristic magnitude for liquid level sensing, then a very importantfactor is the cleanliness of the electrode surface, ensuring a constanttransfer resistance to the liquid. For reliable functioning, suchelectrodes must therefore be cleaned regularly, which means additionalwork and this cannot always be done without damaging the electrodes.

It was therefore the object of the invention to avoid thesedisadvantages and to provide an apparatus by which reliable and accuratesensing of liquid levels could be carried out in a shorter time andindependently of dirtiness of the sensing means.

This problem is resolved according to the invention by an apparatus forsensing levels in containers holding liquids, particularly washingmachines and the like, in which the variable of state ofa sensing means,for example its higher temperature in relation to the temperature of theliquid, is irregularly varied as the level of liquid to be sensedapproaches and reaches the sensing means, in that it is not the absolutevalue of the variable state, but its temporal change, i.e., its rate ofchange, which is used as the characteristic magnitude for sensing thelevel ofthe liquid.

The aforesaid difficulties are therefore eliminated and the level ofliquid is sensed reliably, quickly and accurately. As soon as thesurface of the liquid reaches the level of the sensing means, thevariable of state of this latter starts to change. A considerableadvantage of the apparatus of the invention resides in that it is nolonger necessary to wait until this change is completed, i.e., until thevariable of state has reached its new limit value. Instead, theexistence of a temporal change, which is naturally at its greatest atthe start, is sufficient to cause the sensing means to respond. Ofprimary importance is the tendency of the change, in other words itsdirection, while the amount of the change is of secondary importance.

One embodiment of the apparatus of the invention is thereforecharacterized in that the variable of state is the transfer resistanceor conductance between the two electrodes, one electrode being disposedat a predetermined height, the other electrode being below it,preferably near the bottom of the container.

In this case, therefore, the characteristic magnitude for scanning ofthe level of liquid is the rate of change of the resistance prevailingbetween two electrodes. When the container is empty, this resistance isequal to the insulation resistance, i.e., virtually infinitely great,whereas when in the filled state, it has a lower value which, in thecase of a highly conductive liquid, may fall to practically nil. Withthe method according to the invention, however, it is not the exactvalue of the resistance or conductance which is important, but the factof the temporal change, dirt or deposits being unimpor tant.

An embodiment in which, in per se known manner, the container itselfacts as the counter electrode, is advantageous. This arrangementeliminates the need for a second electrode. lfthe conductivity of theliquid is sufficient, it will for practical purposes be at the potentialof the container walls. This alternative embodiment is however alsofavorable in the case of a nonconductive liquid, when the variable ofstate is the capacitance between the measuring electrode and thecounterelectrode or container.

According to another embodiment of the invention, the variable of stateis the temperature-dependent electrical resistance of a heatedstructural component.

In this case, it is possible to manage with a conventional sensing meanswhile avoiding the sources of error inherent in the conventionalapparatus. Here, too, the essential advantage is that the actuatingdevice is tripped at the moment when there is an appropriate rate ofalteration, rather than when a predetermined switching temperature isreached at the sensing means; in other words, there is virtually nodelay. The sensing signal, therefore, is the differential quotient ofthere sistance or conductance in relation to time. One embodiment of theapparatus provides for the absolute value of the electrical resistanceof the heated component to be used for scanning the temperature of theliquid at the same time as it scans the level of the liquid.

This results in a simplification which at the same time means a savingon cost due to the reduction in components. The saving proves soadvantageous that, beside it, the aforesaid disadvantages of absolutemeasurement become unimportant.

In a further embodiment of the invention, the temperature of the liquidis ascertained by a first sensing means which is not itself heated andwhich is preferably disposed close to the bottom of the container, whilea second heated sensing means is disposed at a predetermined height and,as its ambient changes, in other words as liquid reaches it or leavesit, senses a rate of temperature change.

With such an arrangement, the upper of the two-sensing means serves as alevel switch while the lower one is used as a temperature sensing meansand possibly also as a boil-dry protection. For economic reasons, it isadvantageous if the twosensing means are of the same construction.

According to a further feature of the invention, both sensing means canact on the same arrangement which responds in like or unlike manner tothe two different control magnitudes.

This measure permits further savings. In the case of sensing means withelectrical components, the outlay or circuitry is reduced, because thesensing means can be connected with the same part of the controlcircuit, for example the same amplifier stage.

An advantageous specialized form of the invention provides for thesensing means to be inserted in electrically insulated manner in thecontainer walls, for example in heat-conductive, force-fittedcartridges.

This arrangement permits of favorable heat transfer and simple fitmentto cope with maintenance or repair operations.

The invention further provides for the temperature-dependent resistancesof the two-sensing means to be wired together in a voltage divider witha further electrical component, the tapping of which is coupled tocontrol the electrode of an amplifier, whose output has a measuring anda switching function.

This would make for simple evaluation of the signals transmitted by thesensing means. The measuring function of the amplifier can be achievedby the level of the voltage applied to the amplifier output, while theswitching function is achieved by the response of a suitable device.

It is also expedient for the voltage divider tapping to be connected tothe control electrode along two parallel paths, each of which providespassage for only one control magnitude.

Thus, the two signals fed to the same input are divided again beforebeing amplified and differently affect the control electrode.

It is advantageous for one path to have galvanic coupling and the othercapacity coupling.

Slowly changing signals will therefore reach the control electrode viathe galvanic path, while rapid changes are transmitted through thecapacity coupling. The dimensioning of the components is governed by theanticipated changes or rates of change.

In order to achieve the switching function of the amplifier, it isadvantageous for a glow-discharge tube, tens diode or the like isapplied to the amplifier output in series with a relay, transformer orthe like.

Such a measure ensures, in a simple manner, that no change in thecircuitry occurs until a predetermined striking or breakdown voltage isreached, but that a switching process will immediately be triggered whenthat happens.

An important further feature provided for the tapping of the voltagedivider to be connected to the control electrode of the amplifierthrough an RC link, the coupling resistance being preferably at leastone magnitude higher than the leak resistance of the control electrode.

Such a coupling arrangement can analyze the two different signals in aparticularly elegant manner. However, due to the sharp reduction involtage, it may be necessary to provide two or more amplifier stages.

An essential feature of the invention also resides in that temperaturechanges in only one direction are utilized for switching and controlpurposes, while changes in the other direction are ignored.

This arrangement is very advantageous, particularly in the case ofheated containers, where the switching-on of the heating has to beinterrupted until such a time as a preset level of liquid is reached.Due to the rapid response of the rate in temperature change of thesensing means, such a block is effective virtually without any delay.

Embodiments of the invention are explained hereinafter, furtheradvantages and features of the invention becoming manifest from thedescription and the appended drawings, in which:

FIG. I is a typical heating up curve for a liquid container;

FIG. 2 shows the temperature curve of a sensing means fitted in thecontainer while this is being filled;

FIG. 3 shows the arrangement of the two-sensing means in the containerwall;

FIG. 4 shows, in principle, the temperature curve of container liquidand sensing means;

FIG. 5 shows the coupling of the sensing means signals to an amplifierarrangement;

FIG. 6 shows a special example of such a coupling; and

FIG. 7 shows an example of conductance sensing.

As can be seen in FIG. I, it takes a considerable time for the contentsof a liquid container to become heated. The curve shows the rise oftemperature T during the course of time t, a predetermined temperaturelevel 1",... for example 65 C., also being shown in the diagram. As arule, the contents of the container are heated very slowly, the averageheating-up speed being for example I to 2 C./min.

FIG. 2 shows the temperature curve of a sensing means constructed as aheated electrical resistance, the heating of which is set in motion atthe same time as the liquid container is put into service, in otherwords the first time the latter is filled. In the case of absolute valuesensing, the temperature of the sensing means must have been reached by'1", before the preset filling level is reached. Until at least themoment of time which is designated t,,,, sensing of the filling levelmust remain blocked, to prevent premature response of the control means.The rate of heating of the sensing means is high, for example 60 toC./min. Cooling of the resistance from its normal working temperature,designated T to the response value T takes place through the resistancesheathing, in other words at a slightly slower speed, for example betterthan 45 "a In the case of the differential sensing method, the controlarrangement is ready to function as soon as the temperature of theresistance is slightly above the temperature of the liquid. A coolingprocess then takes place, with a negative Jlldr. Blocking duringpreheating of the resistance (JIId is unnecessary with the differentialquotient method, because the response can. by circuitry measures, bemade easily dependent upon the sign of the differential quotient.

The process to be described later for simultaneous absolute temperaturemeasurement and differential quotient sensing as a boil-dry protectionare exactly the opposite.

The arrangement of the sensing means in the container is shown in FIG.3. It will be seen that the container 1 has, in its wall 2 an uppersensing means 3 and a lower sensing means 4, an electrical resistance 6with terminals 7 being provided inside a sheath 5. In the arrangementillustrated, the upper sensing means 3 functions as a level switch, thelower sensing means 4 as a temperature sensing means and a boil-dryprotection. It is advantageous to use resistances with ahigh-temperature dependence, for example with a negative temperaturecoefficient. Such elements quickly respond to changes in temperaturewith an oppositely oriented resistance variation.

FIG. 4 shows how the lower sensing means 4 according to the inventionfunctions. The curve I0 indicates how the temperature of the liquidchanges as a function of the time, on a basis of the heat supplied.Above it is the sensing means temperature curve ll. Due to the fact thatthe sensing means has its own heating. The temperature of the sensingmeans is always something higher than that of the contents of thecontainer. When the desired temperature of the liquid has reached thetime 1,, therefore the sensing means is at a correspondingly highertemperature.

The instance of the resistance 6 of the lower sensing means 4 beingheated without contact with the liquid, is shown in dash-dotted linesfor the times and in FIG. 4. It will be seen that, in this case, thereis an immediate sharp rise in sensing means temperature as compared withthe progress of curve I! and this irregular temperature change, with itsvery speed, is used according to the invention for switching andcontrolling purposes. The lower sensing means 4, therefore, serves as aquickly responding boil'dry protection because, when the sensing meansis no longer in contact with the liquid, there is an immediately sharpdifferential quotient of the temperature in relation to the time.

The wiring diagram in FIG. 5 shows how the variable sensing meansresistance I5, together with a fixed resistance 16, forms a voltagedivider, the tapping 14 of which emerges into two different conductorsl7 and 18. Conductor 17 is directly, i.e., galvanically coupled to anamplifier l9 (absolute value measurement), while conductor 18 iscapacity coupled via the condenser 20 to the input of the amplifier 22(measurement of the differential quotient). The outputs of amplifier 19and amplifier 22 are taken to a response apparatus 23 which can forexample consist of a glow-discharge tube with a series-connected relay.

FIG. 6 shows a simplified arrangement in which the tapping 14 of thevoltage divider, which is formed by the variable sensing meansresistance and the fixed resistance 16, is connected to an R-C link.This consists of the capacitor and a coupling resistor R the resistancevalue of which is preferably at least one magnitude above the leakageresistance R,,. Steep signal forms are therefore linked to the followingamplifier stage 25 through the condenser 20 while very flat forms arelinked to the same amplifier stage 25 but through the resistance R theamplifier stage 25 is fed via the feed resistance 26 and is for exampletransistorised.

The invention is by no means limited to the use of the circuitsillustrated. Any other circuit or arrangement which permits of thevariable of state and its quotient to be separated may also be used.

As a further embodiment, FIG. 7 shows the differential quotient sensingof the conductance of the liquid by means of sensitive electrodes 27.

Even though the invention has been explained hereinbefore substantiallywith reference to electric sensing means, it must be stressed that othersensing means can also be employed according to the invention. Forexample, it is conceivable that diaphragm capsules may serve as thesensing means, in which case the variable of state will be the pressureor volume and, according to any change thereof in relation to time, sothese will serve as the characteristic magnitude for sensing the levelof contents.

Every possibility of using the rate of change of a variable of state forsensing a level of contents may be described as an application of theinvention.

1 claim:

1. Apparatus for sensing levels in containers holding liquids,particularly washing machines, and the like. which comprises thefollowing:

a. A sensing means for sensing the rate of change of the temperaturegenerated by changing liquid levels;

b. Means for transforming the sensed rate of change into an electricalsignal;

c. Means for transmitting the signal to a responsive receiver;

d. Means for measuring the signal to determine its magnitude; and

e. A utilization means for receiving the measured signal whereby therate of change of the temperature is used to control the liquid in thecontainer.

2. The apparatus of claim 1 wherein the sensing means is positioned at apredetennined height in the container.

3. The apparatus of claim 1 wherein the utilization means is responsiveonly to a rate of change in only one direction.

